Such analytical devices are applied, for example, in process measurements technology or in industrial measurements technology. For example, analytical devices can serve to monitor and optimize the cleaning effectiveness of a clarification plant, to monitor an aeration basin and the outlet of a clarification plant or to control a flocculent. Furthermore, analytical devices can be applied to monitor drinking water or to monitor the quality of food. The content of specific substances, for example, ions such as ammonium, phosphate, silicate or nitrate, or biological or biochemical compounds, e.g. hormones, or also microorganisms in the liquid sample is measured and monitored. The total carbon content (TOC) or the chemical oxygen demand (COD) are other measured variables, which are determined by analytical devices in process measurements technology, especially in the area of water monitoring.
In analytical devices, the sample to be analyzed is frequently mixed with one or more reagents, so that a chemical reaction occurs in the liquid sample. Preferably, the reagents are selected so that the chemical reaction is detectable by means of physical methods, for example, through optical measurements or by means of potentiometric or amperometric sensors or through a conductivity measurement. For example, the chemical reaction can affect a coloring or a color change, which is detectable photometrically, thus through optical means. In this case, the color intensity is dependent on the value of the measured variable to be determined.
In order to automate such analytical methods, for example, in industry, or to monitor a clarification plant or a body of water outdoors, the provision of an automated analytical device, which performs the required analytical method, is required. In addition to a sufficient accuracy of measurement, the most important requirements for such an analytical device are robustness, easy serviceability and the assurance of a sufficient working or environmental safety.
Semi-automatic and automatic analytical devices are known from the state of the art. Thus, for example, DE 102 22 822 A1, DE 102 20 829 A1 and DE 10 2009 029305 A1 describe online analyzers for analyzing measurement samples. Each online analyzer is embodied as cabinet device including a control unit, a liquid supply container for reagents, standards and cleaning liquids, pumps for conveying and dosing the liquid sample and or reagent(s) into a cuvette and a measuring transducer for optical measurements of the liquid sample mixed with the reagent(s) in the cuvette. The reagents are moved through lines from the supply containers into the cuvette. Correspondingly, used liquid from the cuvette is transferred to the waste liquids container.
The liquid supply containers of such an analytical device must be refilled or replaced from time to time. In many analytical methods, liquids, which have only a limited storage life, are used as reagents. The time span, also referred to as the maintenance interval, after which a replacement or a refilling of at least the liquid supply containers containing reagents is required, is frequently not primarily determined by the liquid volume contained in the supply containers and the consumption of reagents by the analytical device, but rather by the limited storage life of the reagents. However, it is desirable to use such an automatic analytical device for as long a period of time as possible without maintenance measures to be performed by operators.
German Offenlegungsschrift DE 195 36 789 A1 proceeds from knowledge that storage life of reagents of an automatic analytical device is frequently reduced by gases, such as oxygen, carbon dioxide or ammonia getting into the supply container. Evaporation can also degrade the analytical results in the case of supply containers open to the atmosphere. In German Offenlegungsschrift DE 195 36 789 A1, a vessel for liquids, which limits both the ability of air to get in and the evaporation of reagents, is provided to improve the storage life of liquids contained in analyzers. The vessel includes a removal opening, starting from which a tube extends into the vessel. A gas exchange occurs between the environment and the liquid in the vessel via the tubes, which should preferably penetrate into the liquid. The size of the removal opening is selected for limiting the ability of air to get in and for the evaporation to be as small as possible.
Moreover, DE 195 36 789 A1 discloses an approach known from the state of the art for lengthening the storage life of liquids. The opening of the supply container is provided with a septum, which is penetrated by a pipetting needle in removal steps. Such an apparatus requires, however, a relatively high mechanical stability of the pipetting needle. Moreover, a pipetting needle is not robust and unsuitable for application in a cabinet device, which is to be applied for monitoring a body of water, even outside in given cases, or in a clarification plant.
It is true that both approaches for lengthening the storage life of the reagents described in DE 195 36 789 A1 bring about an improvement compared to the application of open supply vessels; however, they only lead to a slowing of the degradation of reagents; this degradation is especially caused by contact with air. The contact with air cannot be completely prevented in both cases, so a maintenance free operation of the analytical device over a period of time of many months is not possible. A continuous degradation of the reagents and therewith a continuous worsening of the quality of the analytical results is to be expected, at the least.